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ME456: Mechatronics Systems Design

Chapter 7:Measuring Light with an LED

Prof. Clark J. RadcliffeMechanical Engineering

Michigan State University

http://www.egr.msu.edu/classes/me456/radcliff

LED Light Sensors

• Reference:R. Stojanovic and D.Karadaglic, “Single LED

Takes On Both Light Emitting and LightDetecting Duties”, Electronic Design, Vol. 55,No. 16, 7.19.07

www.electronicdesign.com

LED’s

• Can be both light emitters anddetectors

• When voltage biased,– they emit light– characteristic voltage “diode drop” at a

current of 10-20 mA.• When subjected to light,

– they generate a backwards biasedcurrent proportional to the light strikingthe diode.

Photocurrent

• typical photodiode LED– about 50pA photocurrent.

• The light emitted and the detected– at nearly the same frequency– green-yellow LED emits wavelength 555nM– detects light at 525nm.

• Small photocurrent as a detector– is readily detected by a microcontroller

LED used as a Photodetector

• Analytical Model: Parallel Combination– Current Source (~50pA)

• LED’s response to external light

– Very Small Internal Capacitance (10-15pF )• LED internal structure.

2 Conductors + Dielectric =Capacitor

The Conceptual Model

iR

LED with lightapplied

Equivalentcurrent sourceand parallelcapacitor

2

Analytical Model

• For constant light, iR = constant• Response decreases linearly from VP1(0)

dt!= (t)i C

1- (0) V(t)V R

rP1P1

LED Voltage Vcr vs Photocurrent ir

(Cr = 10 pF)

0.00E+00

1.00E+00

2.00E+00

3.00E+00

4.00E+00

5.00E+00

6.00E+00

0 0.001 0.002 0.003

Time, t

Vcr(

t)

ir=25uA

ir=50uA

Discharge Time

If the LED is initially “charged” to = 5 volts,the time required for the voltage to drop to1/0 threshold voltage (1.4 volts) is directlyrelated to the average photocurrent iR.

Simulated responses show the behavior forCr = 10pF and iR = 25µA and 50 µA

RCTIME Values

At iR = 25µA , t1.4 = 0.7 ms => 3500 countsAt iR = 50µA , t1.4 = 1.4 ms => 7000 counts

LED Voltage Vcr vs Photocurrent ir

(Cr = 10 pF)

0.00E+00

1.00E+00

2.00E+00

3.00E+00

4.00E+00

5.00E+00

6.00E+00

0 0.001 0.002 0.003

Time, t

Vcr(

t)

ir=25uA

ir=50uA

Testing the Sensor

• Pin 2 = 0, Pin 1 is controlled by RCTIME

The Test Code'{$STAMP BS2}'{$PBASIC 2.5}'file:LED_Light_Detecter.bs2'Clark Radcliffe Michigan State University July 31, 2007 'Requires an LED in series with a 220 Ohm resister between pins 2 and 1'Declarationslight VAR WordOUTPUT 1OUTPUT 2LOW 2DO HIGH 1 PAUSE 5 RCTIME 1, 1, light DEBUG CLS, "Light Count = ", DEC5 light PAUSE 200LOOP